The axisymmetric all-polyethylene patellar button is a staple of the modern tricompartmental knee arthroplasty.

The seminal design that appeared in the late 1970's featured a simple dome with a circular base and a central large fixation peg. In the modern era, the most common designs feature a circular base, 3 radially-patterned fixation pegs, and 1 of 3 geometrical variations of an articular surface: a simple dome, an anatomical dome, or a conical dome (as shown left to right in Fig 1).

The simple dome has one key advantage and a few disadvantages. On the plus side, a system featuring a simple dome will tend to have femoral components with a trochlear groove that allows for fairly congruent contact with previously placed simple dome patellas, which can be advantageous at the revision setting when the surgeon prefers not to replace the patellar button. On the minus side, larger diameters of the circular base of the simple dome patella often require a relatively thick implant (and bone resection) in order to attain the  base diameter. The choice of a simple dome often results in a trochlear groove that tends to be fairly wide and less congruent with the more typical articular surfaces of the native patella, and thus both necessitates what is a relatively wide femoral component anterior flange (in the medial-lateral dimension) by contemporary standards and discourages non-resurfacing of the patella. In summary, the simple dome is an ideal choice for the orthopaedist who: i) always resurfaces the patella, ii) is not concerned with covering the resected patellar surface in its entirety, and iii) does a fair number of revision total knees.    

Fig 1. Patellar buttons with simple, modiifed, and conical domes (left to right).

The disadvantages of the simple dome led to the evolution of the anatomic dome. Its strengths address the weaknesses of the simple dome. The anatomic dome would be the patella of choice for the surgeon who: i) sometimes chooses not to resurface the native patella, ii) prefers to cover as much of the resected surface as a circular base will allow (without requiring a thicker bone resection), and iii) either does few revisions involving simple dome patellas, chooses to implant a femoral component with simple dome-style trochlear groove in such cases, or tolerates the articular incongruity between the retained simple dome patella and the anatomical trochlear groove of the newly-placed femoral component.

The third variation of the articular surface seen in contemporary axisymmetric patellar designs is the conical dome, which is really just an extreme example of the anatomic dome. Naturally, its strengths mirror those of the anatomic dome, as do its weaknesses. 

When designing a three-pegged axisymmetric patella implant, another key design parameter is the spacing of the pegs. In some designs, the peg spacing radius remains constant as the base diameter increases, whereas in others the peg spacing radius increases. From a theoretical perspective, it makes sense for the pegs to move farther out as the base diameter (native patella size) increases. However, in practice, there are advantages to maintaining the peg spacing as constant. If the change in peg spacing radius is large between diametrical sizes, a late intraoperative decision to increase/decrease the patella size may require repreparation of the patellar peg holes and relatedly additional bone removal. Furthermore, the change in peg spacing radius will require more surgical drill guides, or at the least a more complex surgical drill guide instrument, to prepare the peg holes versus a system where the peg spacing radius is constant. Thus, it is more more common today for the peg spacing radius to remain constant between sizes than for it to increase from size to size.

The prevalence of the 3-pegged axisymmetric patella implant today is itself an offshoot of its practicality. An anatomic asymmetric patella button with a linear ridge on its articular surface mimicking the medial ridge of the native patella makes the most sense theoretically but would introduce a number of complexities to the instrumentation and surgical technique that few manufacturers or surgeons wish to take on. The future will almost certainly see a rise in the prevalence of axisymmetric anatomic domes relative to the simple dome due to an expected slight to moderate uptick in the percentage of arthroplasties done without patellar resurfacing. Also anticipated is the introduction of designs that seek to minimize the volume of bone resected from the patella when a patella implant is placed. The metal-backed cementless patellar button will also re-emerge as a somewhat popular patellar resurfacing alternative as surgeons become more comfortable placing tibial and femoral total knee components without cement.